Plastic molding and method and apparatus for producing the same by injection molding

ABSTRACT

A plastic molding for use in an optical device, a method of producing the molding by injection molding, and an apparatus for practicing the method are disclosed. Only a part of a molding expected to sink is surely caused to sink while, e.g., a mirror surface is surely transferred to a desired part of the molding.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a lens, mirror, prism or similarplastic molding produced by injection molding and included in an opticaldevice, e.g., a copier, laser printer, facsimile apparatus or similarimage forming apparatus, and a method and an apparatus for producing thesame. More particularly, the present invention is concerned with aplastic molding having, e.g., mirror surfaces and a fine undulationpattern transferred thereto with high accuracy by injection molding, anda method and apparatus for producing the same.

[0002] For injection molding, it is a common practice to use a moldassembly including a mold surface forming a cavity having a preselectedvolume, a transfer surface formed on the mold surface for transferring amirror surface to a molding, and a gate open at the mold surface andhaving a preselected opening area. Molten resin is injected into thecavity via the gate and then cooled. The resulting molding is taken outby opening the mold assembly. While such a molding, particularly amirror, lens, prism or similar optical element, is required to have anaccurate mirror surface and a uniform refractive index, the mirrorsurface needing a high surface accuracy is caused to sink because themolten resin contracts at the time of solidification.

[0003] Injection molding methods for solving the above problem aretaught in, e.g., Japanese Patent Laid-Open Publication Nos. 3-128218,8-234005, 3-151218, and 3-281213 (Prior Art 1 hereinafter). In prior Art1, a non-transfer surface or mold surface facing a transfer surfaceformed with, e.g., a mirror surface is roughened, or surface treatmentfor lowering wettability is effected, or use is made of a porousmaterial. Injection is stopped just before a cavity is filled up withmolten resin. Then, the molten metal is solidified by cooling withoutany dwelling. As a result the roughened surface is caused to sink due toa difference in adhering force between the molten resin, the transfersurface, and the roughened surface. This prevents the mirror fromsinking. Alternatively, an overflow portion for receiving excess moltenresin is located outside of the cavity. When the overflow portion beginsto be filled, injection is stopped. Then, the molten resin is solidifiedby cooling without any dwelling. This also allows the roughened surfaceto sink due to a difference in adhering force between the resin, thetransfer surface, and the roughened surface.

[0004] An injection molding method disclosed in Japanese PatentLaid-Open Publication No. 2-175115 (Prior Art 2 hereinafter) injectsmolten metal into a cavity in which a porous member communicated to acompressed gas is provided on a mold surface contacting the non-transfersurface of a molding. While dwelling and cooling are under way after theinjection of the molten resin, air is fed to the non-transfer surface ofthe molding via the porous member. With this method, it is possible tocause a side of a cylindrical thin lens to sink.

[0005] Japanese Patent Laid-Open Publication No. 6-304973 (Prior Art 3hereinafter) proposes an injection molding method in which anon-transfer surface is communicated to the outside air via a vent hole.During an interval between the beginning and the end of injection ofmolten resin into a cavity, a pressure difference is generated betweenthe transfer surface and the non-transfer surface of the resin. As aresult, the non-transfer surface of the resin is caused to sink.Specifically, air is brought into contact with the molten resin otherthan the mirror transferred from the transfer surface via the vent holeand a bore communicated thereto, so that the cooling speed of the resinis lowered. At the same time, a preselected air pressure is fed to thevent hole in order to generate a preselected pressure difference betweenthe mirror portion of the resin and the vent hole. This allows only theportion of the resin facing the vent hole to sink, i.e., prevents themirror portion from sinking. In addition, because only the vent holeportion of the resin sinks, a molding can be produced by simple controlover the amount of the resin to be injected into the cavity and withoutany strain being generated in the resin. The resulting molding istherefore free from an internal strain and provided with an accuratemirror surface.

[0006] Prior Art 3 further teaches that the vent hole may becommunicated to a compressor so as to apply a preselected air pressureto the vent hole portion of the resin. With this configuration, it ispossible to generate any desired pressure difference between the mirrorsurface portion and the vent hole portion of the resin, thereby causingthe vent hole portion to sink. In addition, the pressure difference isreadily adjustable in order to further enhance the accuracy of themirror surface without any internal strain.

[0007] Japanese Patent Laid-Open Publication No. 6-31596 (Prior Art 4hereinafter) teaches an injection molding method causing thenon-transfer surface of resin to sink. In accordance with this method,the transfer surface of a mold heated to and held at a high temperature.The transfer surface side of the resin is heated to a high temperatureuntil the injection of molten resin into a cavity ends.

[0008] However, Prior Art 1 relying on the roughened surface, surfacetreatment or porous material results in an expensive mold assembly.Moreover, stopping the injection just before the cavity is filled upwith the molten metal is extremely difficult. Should the timing forstopping the injection be deviated, the relation in adhering forcebetween the transfer surface and the roughened surface would be invertedand would thereby cause the mirror surface to sink or result in shortresin. In addition, because sinking cannot be provided withdirectionality and because setting the molding conditions is difficult,the configuration of the molding is critically limited. The filling ofthe molten resin may be stopped at any time lying in a broader range.However, the overflow portion formed integrally with the molding must beremoved by an extra step. Moreover, should the opening area of the gatefor feeding the molten resin to the overflow portion be excessivelysmall, the relation in adhering force between the transfer surface andthe roughened surface would also be inverted and would thereby cause themirror surface to sink. Should the opening area be excessively small,the molten resin would become short.

[0009] Prior art 1 can implement a mirror or similar optical elementneeding a single mirror surface because it roughens the mold surfacefacing the transfer surface. However, Prior Art 1 cannot produce a lens,prism or similar optical element because the number and positions ofmirror surfaces are limited. In addition, the relation in adhering forceis inverted and causes the mirror surface to sink, depending on thematerial constituting the transfer surface and roughened surface and thekind of the resin.

[0010] Prior Art 2 increases the cost of the mold assembly due to theporous member and sophisticates control over the configuration of theporous member. Specifically, if the effect of the porous member isexcessive, it not only admits the molten metal thereinto, but alsoobstructs the parting of the molding. This is particularly true when theporous portion of the porous member extends inward over the wall of themold. Further, because the compressed gas is fed to the non-transfersurface of the molding via the porous member during the previouslystated interval, a pressure difference is maintained between thenon-transfer surface and the transfer surface of the resin duringcooling. As a result, the internal strain remains in the resultingmolding after the opening of the mold. The residual pressure not onlylowers the accuracy of the transfer surface, but also causes the entiremolding to deform.

[0011] Prior Art 3 generates a pressure difference between the transfersurface and the non-transfer surface of the resin during the intervalmentioned earlier. This also brings about the problem stated above inrelation to Prior Art 2. Prior Art 4 maintains the transfer surface ofthe mold at a high temperature and heats the transfer surface side ofthe resin to a high temperature during the previously mentionedinterval. This is also undesirable in the above respect.

SUMMARY OF THE INVENTION

[0012] It is therefore an object of the present invention to provide aplastic molding capable of surely sinking only at a desired portionthereof and being surely provided with a mirror surface in anotherdesired portion thereof, and a method and an apparatus for producing thesame by injection molding.

[0013] It is another object of the present invention to provide aninexpensive and least deformable plastic molding capable of surelyguiding sinking to a non-transfer surface thereof and therefore having ahighly accurate transfer surface.

[0014] In accordance with the present invention, in a molding producedby an injection mold assembly having a pair of molds including a moldsurface forming a cavity having a preselected volume, at least onetransfer surface for transferring a mirror surface formed on the moldsurface to the molding, and a gate for filling the cavity with a moltenmaterial by injection, and by injecting the molten material into thecavity via said gate and then cooling the molten material, the injectionmold assembly includes at least one vent hole having a preselectedopening area, and at least one bore communicated to the vent hole forapplying a preselected air pressure to the molding. A step portion isformed on the mold surface between the vent hole and the transfersurface.

[0015] Also, in accordance with the present invention, in an injectionmolding method for producing a molding by using a mold assembly having apair of molds including a mold surface forming a cavity having apreselected volume, at least one transfer surface for transferring amirror surface formed the mold surfaces to the molding, and a gate forfilling the cavity with a molten material by injection, and by injectingthe molten material into the cavity via the gate and then cooling themolten material, the mold surface is formed with, outside of thetransfer surface, at least one vent hole having a preselected openingarea and at least one bore communicated to the vent hole for applying apreselected air pressure to the molding material. The air pressure iscontinuously generated via the vent hole even after the pressure of themolding material in the cavity has dropped to zero.

[0016] Further, in accordance with the present invention, a moldassembly has a pair of molds including a mold surface forming a cavityhaving a preselected volume, at least one transfer surface fortransferring a mirror surface formed on the mold surface to the molding,and a gate for filling the cavity with a molten material by injection,and injects the molten material into the cavity via the gate and thencools the molten material. The mold surface is formed with, outside ofthe transfer surface, at least one vent hole having a preselectedopening area and at least one bore communicated to the vent hole forapplying a preselected air pressure to the molding material, and atleast one exhaust hole located at a position adjoining the vent hole,but not facing the transfer surface.

[0017] Moreover, in accordance with the present invention, a method ofproducing a plastic molding begins with the step of preparing a moldassembly including at least one transfer surface and at least onenone-transfer surface formed on a surface other than the transfersurface. The transfer surface and non-transfer surface forms at leastone cavity. Molten resin heated to a temperature above a softening pointthereof is injected into the cavity. A resin pressure is caused to acton the transfer surface to thereby cause the resin to adhere to thetransfer surface, and then the resin is cooled to a temperature belowthe softening point. The mold assembly is opened in order to allow theresulting molding to be taken out. The temperature of at least onenon-transfer surface of the resin is lowered below the temperature ofthe resin on the transfer surface during an interval between thebeginning and the end of injection of the resin into the cavity.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The above and other objects, features and advantages of thepresent invention will become more apparent from the following detaileddescription taken with the accompanying drawings in which:

[0019]FIG. 1 is a fragmentary view showing a conventional injection moldassembly;

[0020]FIG. 2 is a plan view showing a specific vent hole formed in themold surface of a sink insert included in the mold assembly of FIG. 1;

[0021]FIG. 3A is a perspective view showing a specific molding producedby the mold assembly of FIG. 1;

[0022]FIG. 3B is a side elevation of the molding of FIG. 3A;

[0023]FIG. 3C is a section along line A of FIG. 3B;

[0024]FIG. 4A is a side elevation showing a specific molding produced byinjection molding with an air stream flowing toward the referencesurface of a cavity;

[0025]FIG. 4B is a section along line A of FIG. 4A;

[0026]FIG. 5A is a top plan view showing a specific molding produced byinjection molding and caused to sink as far as its mirror surfaceportion;

[0027]FIG. 5B is a side elevation of the molding shown in FIG. 5A;

[0028]FIG. 5C is a section along line A of FIG. 5B;

[0029]FIG. 6 shows a relation between the molding produced by the moldassembly of FIG. 1 and the position of a vent hole;

[0030]FIG. 7A is a perspective view showing a molding and an injectionmold assembly representative of a first embodiment of the presentinvention;

[0031]FIG. 7B is a fragmentary section of the mold assembly shown inFIG. 7A;

[0032]FIG. 8 is a perspective view of a molding representative of asecond embodiment of the present invention;

[0033]FIG. 9A is a perspective view of a molding representative of athird embodiment of the present invention;

[0034]FIG. 9B is a section of the third embodiment in plane A1 of FIG.9A;

[0035]FIG. 9C is a section of the third embodiment in plane A2 of FIG.9A;

[0036]FIGS. 10A and 10B are sections each showing a particularconfiguration of steps included in the third embodiment in the directionof height;

[0037]FIG. 11A is a perspective view of a molding representative of afourth embodiment of the present invention;

[0038]FIG. 11B is a section in plane A of FIG. 11A;

[0039]FIG. 12A is a perspective view of a molding representative of afifth embodiment of the present invention;

[0040]FIG. 12B is a section in plane A of FIG. 12A;

[0041]FIG. 13A is a perspective view of a molding representative of asixth embodiment of the present invention;

[0042]FIG. 13B is a section in plane A of FIG. 13A;

[0043]FIG. 14A is a perspective view of a molding representative of aseventh embodiment of the present invention;

[0044]FIG. 14B is a section in a plane A of FIG. 14A; FIG. 15 is asection of a molding representative of an eighth embodiment of thepresent invention and including tapered steps;

[0045]FIGS. 16A and 16B are sections each showing a particularconfiguration of a molding representative of a ninth embodiment of thepresent invention;

[0046]FIG. 17 shows the variation of the internal pressure of moltenresin existing in a cavity occurring from the beginning to the end ofcooling of the resin, and a timing for switching an air pressure fed viaa vent hole;

[0047]FIG. 18 is a section of a injection mold assembly representativeof an eleventh embodiment of the present invention;

[0048]FIG. 19 is a perspective view showing a positional relationbetween a vent hole and an exhaust hole included in the eleventhembodiment;

[0049]FIG. 20 is a section of an injection mold assembly representativeof a twelfth embodiment of the present invention;

[0050]FIG. 21 is a perspective view showing the position of an exhausthole formed in an injection mold assembly representative of a thirteenthembodiment of the present invention;

[0051]FIG. 22 is a perspective view showing a modification of thethirteenth embodiment;

[0052]FIG. 23A is a perspective view of a plastic molding representativeof a fifteenth embodiment of the present invention;

[0053]FIG. 23B is a side elevation showing a sinking region to occur inthe fifteenth embodiment;

[0054]FIG. 23C is a section along line A-A of FIG. 23B;

[0055]FIG. 24A is a section showing one half of the fifteenthembodiment;

[0056]FIG. 24B is a perspective view showing cavity inserts included inthe fifteenth embodiment;

[0057]FIG. 25A is a section as seen in a direction X of FIG. 24B;

[0058]FIG. 25B is a section as seen in a direction Y of FIG. 24B;

[0059]FIG. 26 is a section showing a method and an apparatus forproducing a plastic molding representative of a sixteenth embodiment ofthe present invention;

[0060] FIGS. 27A-27D show a specific procedure available with thesixteenth embodiment;

[0061] FIGS. 28A-28D show another specific procedure available with thesixteenth embodiment;

[0062]FIG. 29 is a top plan view showing a method and an apparatus forproducing a plastic molding representative of a seventeenth embodimentof the present invention;

[0063]FIGS. 30A and 30B show another specific procedure available withthe seventeenth embodiment;

[0064]FIG. 31 is a top plan view showing a method and an apparatus forproducing a plastic molding representative of an eighteenth embodimentof the present invention;

[0065]FIG. 32A is a perspective view showing cavity inserts included inthe eighteenth embodiment;

[0066]FIG. 32B is a section as seen in a direction X of FIG. 32A;

[0067]FIG. 32C is a section as seen in a direction Y of FIG. 32A;

[0068]FIGS. 33A and 33B show another specific procedure available withthe eighteenth embodiment;

[0069] FIGS. 34A-34C shows a procedure following the procedure of FIG.32B; and

[0070]FIG. 35 shows a modification of the eighteenth embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0071] To better understand the present invention, brief reference willbe made to the injection molding method taught in Prior Art 3 mentionedearlier. As shown, a mold assembly is made up of a stationary mold 11and movable mold 12, mirror pieces 13 and 14, a reference insert 15, anda sink insert 16 forming a cavity 17 having a preselected volume incooperation. The mirror pieces 13 and 14 respectively have transfersurfaces or mold surfaces Ml and M2 for transferring mirror surfaces toa molding. The reference insert 15 has a mold surface defining thereference surface (C surface) of a molding. The sink insert 16 has amold surface implementing a surface for causing a molding to sink (Bsurface). Molten resin or similar molten molding material 20 is injectedinto the cavity 17 via a gate, not shown. The mold surface of the sinkinsert 16 is formed with a vent hole 18 having a preselected openingarea, and a bore 19 communicated to the vent hole 18. Air underpreselected pressure is fed to the material 20 via the bore 19 and venthole 18.

[0072]FIG. 2 shows a specific configuration of the vent hole 18 formedin the mold surface (B surface) of the sink insert 16. To feed air tothe material 20, use may be made of natural draft utilizing a pressuredifference between the mirror portion of the material 20 and the venthole portion, or forced draft generating a desired pressure differencebetween the two portions with a compressor, not shown, communicated tothe hole 18.

[0073] In the mold 10, the vent hole 18 is positioned at the side of amolding which is expected to sink. When air is fed to the cavity 17 viathe bore 19 and vent hole 18, sinking successfully occurs in theexpected surface of the molding. In addition, the mirror surfaces of themirror pieces 13 and 14 are desirably transferred to the molding. Themolding therefore suffers from a minimum of internal strain.

[0074] A specific molding 21 produced by the mold 10 assembly is shownin FIGS. 3A-3C. As shown, the molding is implemented as a rectangularlens having mirror surfaces (optical surfaces) 22 and 23 transferredfrom the mirror surfaces M1 and M2 of the mirror pieces 13 and 14,respectively. FIG. 3 shows the surface 24 of the lens 21 to be caused tosink (B surface); a sinking area is indicated by crosshatching. Asshown, desired sinking is caused to occur on the B surface 24 of thelens 21. As a result, the mirror surfaces 22 and 23 are desirablytransferred to the lens 21, reducing the internal strain of the lens 21.

[0075] However, the injection mold assembly 10 has the followingproblems left unsolved. If the various mold parts 11-16 constituting themold 10 lack in accuracy either individually or in combination, a gap dis formed between the parts, as shown in FIG. 1. Then, air is likely toflow into the cavity 17 via the gap d and prevent the desired surfacefrom sinking. FIGS. 4A and 4B are views similar to FIGS. 3A and 3B,showing a lens 21 molded with an air stream flowing into the referencesurface side (C surface) of the cavity 17. As shown, the lens 21 failsto sink to a desired degree or practically fails to sink at its expectedsurface (B surface). In the worst case, a C surface 25 is caused to sinkand looses surface accuracy as a reference surface.

[0076] Further, when air is introduced into the cavity 17 via the venthole 18 by either natural draft or forced draft, it is apt to reach themirror surfaces 22 and 23 and cause them to sink also, depending on theamount of resin filled in the cavity 17 or the amount of air. A lens ormolding 21 caused to sink as far as its mirror surfaces 22 and 23 isshown in a top plan view in FIG. 5A, in a side elevation in FIG. 5B, andin a section along line A of FIG. 5B in FIG. 5C. As shown, a sunk region27 formed in the B surface 24 extends even to the mirror surface 22 andintroduces a strain in the mirror surface 22, thereby deteriorating theability of the lens.

[0077] Preferred embodiments of the present invention will be describedhereinafter. It is to be noted that reference numerals designating thestructural elements of each embodiment are independent of the others,i.e., identical reference numerals do not always designate identicalstructural elements.

1st Embodiment

[0078]FIG. 6 shows the positional relation between the conventionalmolding 21 and the vent hole 18 of the mold assembly 10. The relationshown in FIG. 6 brings about the problem discussed with reference toFIGS. 5A-5C. A first embodiment of the present invention eliminates sucha problem by providing a molding with steps between a vent hole andmirror surfaces.

[0079] Specifically, FIG. 7A shows a specific molding 1 representativeof the first embodiment. FIG. 7B shows a part of an injection moldassembly 10 for producing the molding 1. Basically, the mold assembly 10is similar to the conventional mold assembly 10, FIG. 1, and has itsstructural elements designated by the same reference numerals. Thedifference is that, as shown in FIG. 7B, the mold assembly 10 of theembodiment includes steps formed in a cavity 17 between a vent hole 18and mirror surfaces and complementary in configuration to steps 6 to beformed on the molding 1.

[0080] More specifically, the molding 1 is implemented as a rectangularlens having two mirror surfaces (optical surfaces) 2 and 3 playing therole of lens surfaces. The lens 4 includes a surface 4 to be caused tosink (B surface). The steps 6 are formed on the surface 4 between thevent hole 18 and the opposite mirror surfaces 2 and 3, isolating themirror surfaces 2 and 3 from the vent hole 18.

[0081] As shown in FIG. 7B, the mold assembly 10 includes the stepscomplementary in configuration to the steps 6 and formed on the moldsurface between the vent hole 18 and the mirror surfaces. Just aftermolten resin or molding material 20 has been injected into the cavity17, air is forced out of the cavity 17 via the vent hole 18 and a bore19 due to the high internal pressure of the resin 20. The internalpressure of the resin 20 sequentially decreases as the resin 20 iscooled. When the pressure of the resin 20 decreases below theatmospheric pressure or below a compression pressure (when a compressoris communicated to the vent hole 18 via the bore 19), air begins to flowinto the cavity 17 via the vent hole 18, causing the resin 20 to sinkaway from the vent hole 18. Should the steps 6 be absent on the resin20, the resin 20 might sink as far as its mirror surfaces. In theillustrative embodiment, the steps 6 isolating the mirror surfaces 2 and3 from the vent hole 18 tend to contract toward each other, as indicatedby arrows in FIG. 7B. However, the steps of the mold assembly 10interfere with the steps 6 and prevent them from contracting. As aresult, the resin 20 and mold assembly 10 remain in close contact witheach other and prevent sinking from proceeding over the steps 6, i.e.,confine it to the region between the steps 6. The lens 1 is thereforecaused to sink only in its expected portion and surely formed with themirror surfaces 2 and 3 by transfer.

2nd Embodiment

[0082]FIG. 8 shows a molding 1 representative of a second embodiment ofthe present invention and produced by injection molding. The molding 1is also implemented as a lens similar in configuration to the lens ofFIG. 7A. As shown, a step 6 is formed on the B surface 4 of the lens 1adjoining the vent hole, not shown, such that the step 6 surrounds thevent hole. The step 6 surrounding the vent hole prevents air fromturning round and allows the sinking region to be controlled morepositively than in the first embodiment.

3rd Embodiment

[0083] FIGS. 9A-9C show a molding 1 representative of a third embodimentof the present invention. The molding 1 is also implemented as a lenssimilar in configuration to the lens of FIG. 7A. As shown, a step 6 isformed on the side (B surface) 4 of the molding adjoining the vent holenot shown. The step 6 is substantially similar in configuration to thecontour of the side 4. Specifically, when the molding 1 is a rectangularlens, the step 6 surrounds the vent hole, not shown, complementarily tothe contour of the side of the lens. A sinking region can therefore becontrolled in the same ratio as the sectional area of each section ofthe molding 1 (section A1 or A2). This successfully uniforms theinternal strain and surface accuracy and thereby enhances the accuracyof the lens 1.

[0084] As shown in FIG. 10A, the steps 6 of the first to thirdembodiments each is implemented by projections in the direction ofheight. Alternatively, as shown in FIG. 10B, the steps 6 may beimplemented by recesses formed in the molding 1. The recesses are alsosuccessful to control the sinking region. When the steps 6 areimplemented by such recesses, the mold assembly 10 will be formed withsteps in the form of projections around the vent hole 18.

4th Embodiment

[0085]FIGS. 11A and 11B show a molding 1 representative of a fourthembodiment of the present invention. As shown, steps 6 are formed on thesurfaces 2 and 3 of the molding 1 which are expected to sink. Thisconfiguration also prevents air from reaching the mirror surfaces 2 and3 via the vent hole, not shown, because the steps 6 of the molding 1 andthose of the mold assembly, not shown, remain in close contact with eachother.

5th Embodiment

[0086]FIGS. 12A and 12B show a molding 1 representative of a fifthembodiment of the present invention. As shown, a step 6 is formed on apart of a mirror surface 2 or 3 expected to sink. Specifically, when itis known that air will turn round to a part of the mirror surface 2 or 3and cause it to sink beforehand, the step 6 may be formed only in such apart of the mirror surface. This configuration saves cost when a moldassembly, not shown, is formed with a step.

6th Embodiment

[0087]FIG. 13A shows a molding 1 representative of a sixth embodiment ofthe present invention. FIG. 13B is a section in plane A of FIG. 13A. Asshown, steps 6 are formed on opposite edges of the mirror surface 2contiguous with the B surface 4 and C surface 5, respectively. Likewise,steps 6 are formed on opposite edges of the other mirror surface 3contiguous with the B surface 4 and C surface 5, respectively. As shownin FIG. 1, when the mold assembly 10 is not accurate, air is apt toenter the cavity 17 via an unexpected portion. As a result, as shown inFIGS. 4A and 4B specifically, air is likely to flow into the referencesurface (C surface) side of the cavity 17 and turn round to the mirrorsurfaces 2 and 3 to cause them to sink. In the illustrative embodiment,the steps 6 formed on both longitudinal edges of the mirror surface 2and those of the mirror surface 3 prevent air from turning round to themirror surfaces 2 and 3 and causing them to sink.

7th Embodiment

[0088]FIG. 14A shows a molding 1 representative of a seventh embodimentof the present invention. FIG. 14B is a section in plane A of FIG. 14A.As shown, the molding 1 is identical with the molding shown in FIG. 13Aexcept that the steps 6 facing each other on each of the mirror surfaces2 and 3 are replaced with a single step 6 surrounding the mirror surface2 or 3. Such steps 6 can obstruct air more positively and can thereforeprevent the mirror surfaces 2 and 3 from sinking more positively.

8th Embodiment

[0089] To obstruct air tending to reach the mirror surfaces 2 and 3 ofthe molding 1, the steps shown in FIG. 13B or 4B suffice. FIG. 15 showsa molding configured to be easily separable from a mold. As shown, thesteps 6 each is provided with a draft or draught in the direction ofheight h. With such steps 6, the molding can be easily separated from amold assembly while preserving its accuracy.

9th Embodiment

[0090]FIGS. 16A and 16B each shows a particular molding representativeof a ninth embodiment of the present invention. As shown, steps 6 facingeach other at both edges of each mirror surface 2 or 3 are provided witha triangular cross-section (FIG. 16A) or an arcuate cross-section (FIG.16B).

[0091] This not only enhances the parting ability of the molding, butalso simplifies the procedure for forming steps in, e.g., the mirrorpieces 13 and 14.

[0092] In each of the eighth and ninth embodiments, the steps 6 eachhave a height h greater than 0.1 mm inclusive. Experiments showed thatheights h greater than 0.1 mm inclusive can sufficiently obstruct air.

10th Embodiment

[0093] This embodiment relates to a method of forming a molding and willbe described with reference to FIG. 7B. First, the movable mold 12carrying the mirror piece 14 and reference insert 15 therewith isbrought into contact with the stationary mold 11 loaded with the othermirror insert 13 and reference insert 15. As a result, the mold surfacesof the molds 11 and 12 form the cavity 17 having a preselected volume. Agate, not shown, is formed in a mold surface, not shown, of the moldassembly 10 in order to inject the molten resin 20 into the cavity 17. Aconventional filling machine, not shown, is connected to the gate inorder to fill the cavity with the molten resin 20 by injection.

[0094] Just after molten resin or molding material 20 has been injectedinto the cavity 17, air is forced out of the cavity 17 via the vent hole18 and a bore 19 due to the high internal pressure of the resin 20. Theinternal pressure of the resin 20 sequentially decreases as the resin 20is cooled. When the pressure of the resin 20 decreases below theatmospheric pressure or below a compression pressure (when a compressoris communicated to the vent hole 18 via the bore 19), air begins to flowinto the cavity 17 via the hole 18, causing the resin 20 to sink awayfrom the hole 18. At this instant, the steps 6 isolating the mirrorsurfaces 2 and 3 from the vent hole 18, as shown in, e.g., FIG. 7A, tendto contract toward each other, as indicated by arrows in FIG. 7B.However, the steps of the mold assembly 10 interfere with the steps 6and prevent them from contracting. As a result, the resin 20 and mold 10assembly remain in close contact with each other and prevent sinkingfrom proceeding over the steps 6.

[0095]FIG. 17 shows how the internal pressure of the resin 20 variesfrom the time when the resin 20 begins to be filled in the cavity 17 tothe time when it is fully cooled off. In the case where air underpressure is fed via the vent hole 18, its pressure is switched in amanner also shown in FIG. 17. As shown, in the illustrative embodiment,air is continuously fed even after the internal pressure of the resin 20has been lowered to zero, generating air pressure in the vent holeportion. Experiments showed that the air pressure continuously generatedeven after the drop of the resin pressure to zero allows the sinkingregion to be surely controlled.

[0096] More specifically, the resin 20 remains in close contact with themold 10 until the internal pressure of the resin 10 drops to zero, andsinking occurs thereafter. It is therefore necessary to apply the airpressured for some more period of time after the internal pressure hasdropped to zero. It was found that when the molding 1 is implemented asa lens, as shown and described, the sinking region can be controlled ifthe air pressure is continuously applied for more than 5 seconds evenafter the drop of the internal pressure of the resin 20 to zero. The airpressure should preferably be higher than the atmospheric pressure(about 0.1 MPa) inclusive, but lower than 2 MPa inclusive.

[0097] The first to tenth embodiments shown and described achieve thefollowing various unprecedented advantages.

[0098] (1) In a molding formed by an injection mold in which a pressuredifference or an air pressure is generated between mirror surfaceportions corresponding to the mirror surfaces of a molding material anda vent hole portion corresponding to a vent hole in order to cause thematerial to sink, a step is formed in a cavity between the vent hole anda mirror surface portion. When the material or resin is cooled, the stepprevents the resin from contracting over the step and thereby guaranteesadhesion of the portions of the material other than a surface expectedto sink and the mold. This prevents sinking from proceeding over thestep and thereby confines it to a region delimited by the step.

[0099] (2) The step is provided on the surface of the molding facing thevent hole, so that the sinking region can be confined to such a surface.

[0100] (3) Two steps are formed in such a manner as to isolate the venthole and the mirror surface portions, so that sinking is prevented fromextending to the mirror surfaces.

[0101] (4) The step is formed to surround the vent hole in order toprevent air from turning round to the surface portions. This allowssinking to be confined to the region delimited by the step and therebyprevents sinking from extending to the mirror surfaces.

[0102] (5) When the step is similar in configuration to the contour ofthe side of the molding facing the vent hole, the sinking region can becontrolled in the same ratio as the sectional area of the molding. Thisuniforms the internal strain and surface accuracy of the molding andtherefore enhances the accuracy of the molding.

[0103] (6) The sinking region can be controlled both when the step orsteps of the molding are implemented as projections and when they areimplemented as recesses.

[0104] (7) The step or steps prevent air from reaching the mirrorsurface portions via the vent hole and thereby protects the mirrorsurface portions from sinking.

[0105] (8) The steps configured to face with each other at oppositelongitudinal edges of each mirror surface obstruct air coming in throughthe vent hole or any other portion of the mold. This also surelyprotects the mirror portions from sinking.

[0106] (9) The steps each is configured to surround the associatedmirror surface portion. This prevents air from reaching the mirrorsurfaces more positively and obviates the sinking of the mirror surfaceportions more positively.

[0107] (10) The steps are tapered in order to confine the sinking to thearea delimited by the steps. In addition, the taper enhances the partingability of the molding from the mold assembly.

[0108] (11) The steps are provided with a triangular or an arcuatecross-section in order to enhance the parting ability of the molding andto facilitate the formation of steps in the mold assembly.

[0109] (12) In a method of forming a molding of the kind described, anair pressure is continuously generated via the vent hole even after theinternal pressure of the resin in the cavity has dropped to zero so asto control the sinking region more positively.

11th Embodiment

[0110]FIG. 18 shows an injection mold assembly representative of aneleventh embodiment of the present invention. As shown, a sink insert 5is located at a position where sinking is expected to occur. The sinkinsert 5 is formed with a vent hole 7, a bore 8 communicated to the venthole 7, and a pair of exhaust holes 20 respectively positioned above andbelow the vent hole 7 and bore 8. FIG. 19 shows a positional relationbetween the vent hole 7 and the exhaust holes 20.

[0111] In the illustrative embodiment, just after molten resin ormolding material 10 has been injected into a cavity 6, it is difficultfor air 9 fed under pressure via the vent hole 7 to enter the cavity 6.The internal pressure of the resin 10 sequentially decreases as theresin 10 is cooled. When the pressure of the resin 10 decreases belowthe pressure of the compressed air 9 delivered to the vent hole 7, theair 9 begins to flow into the cavity 6 via the vent hole 7. As a result,the portion of the resin 10 corresponding to one side 15 of a molding11, FIG. 19, and facing the vent hole 7 begins to sink (X4) away fromthe inner periphery of the cavity 6. The compressed air 9 introducedinto the cavity 6 hits against the resin 10 and then discharged from thecavity 6 via the exhaust holes 20. That is, the compressed air 9 isprevented from turning round to the upper surface 12 and lower surface13 of the molding 11 which should turn out mirror surfaces. If desired,a machine for forced exhaustion may be connected to the bore 8 in orderto promote more effective discharge of the compressed air 9. When themold assembly is used to form, e.g., a lens of resin applicable to animage forming apparatus or similar optical apparatus, the exhaust holes20 should be 0.001 mm to 0.5 mm wide (vertical dimension in FIG. 18).With such a width, the exhaust holes 20 allow a minimum of resin toenter it and thereby frees the molding 11 from burrs.

12th Embodiment

[0112]FIG. 20 shows a mold representative of a twelfth embodiment of thepresent invention. As shown, this embodiment is identical with theeleventh embodiment except that the exhaust holes 20 are implemented byporous members 21.

13th Embodiment

[0113]FIG. 21 shows the position of an exhaust hole formed in aninjection mold assembly representative of a thirteenth embodiment of thepresent invention. As shown, a continuous exhaust hole 22 is formed tosurround the vent hole 7. The exhaust hole 22 may also be implemented bythe porous member 21 in order to simplify the configuration of theinsert 5. FIG. 20.

14th Embodiment

[0114]FIG. 22 shows a fourteenth embodiment of the present inventionwhich is a modification of the thirteenth embodiment. As shown, thisembodiment is identical with the thirteenth embodiment except that anexhaust hole 23 is similar configuration to the contour of the side ofthe molding 11 which is expected to sink. Again, the exhaust hole 23 maybe implemented by the porous member 21 in order to simplify theconfiguration of the insert 5, FIG. 20.

[0115] The eleventh to fourteenth embodiments shown and described havethe following unprecedented advantages.

[0116] (1) At least one exhaust hole is formed in the vicinity of a venthole used to feed air under pressure for sinking. The exhaust holedischarges air caused sinking to occur in the vicinity of the vent holeto the outside of a mold assembly before it reaches portions expected toform mirror surfaces. Therefore, air is prevented from reaching portionsother than the portion to sink, so that the shape of the mold assemblyis surely transferred to the other portions of the molding.

[0117] (2) A single exhaust hole surrounds the vent hole and dischargesair caused to sinking to occur smoothly to the outside of the moldassembly. This guides air only to the portion of the molding expected tosink more positively.

[0118] (3) The or each exhaust hole is implemented by a porous member.Therefore, particularly when a single exhaust hole surrounds the venthole, the member formed with the holes is simple in structure.

[0119] (4) Air is forcibly discharged via the exhaust holes, so that aircaused sinking to occur in the cavity can be discharged more smoothly.

[0120] (5) The exhaust hole has an opening width as small as 0.001 mm to0.5 mm and prevents a molding material from entering it. This frees theresulting molding from burrs.

15th embodiment

[0121] FIGS. 23A-23C show a plastic molding formed by a methodrepresentative of a fifteenth embodiment of the present invention. Themolding may be implemented not only as a lens but also as a mirror,prism or similar optical device. As shown, a molding 1 has mirrorsurfaces or transfer surfaces 1 a and 1 b on its top and bottom,respectively. In addition, the molding 1 has a reference surface ornon-transfer surface 1 c at one side and a sink surface or non-transfersurface 1 d at the other side. The reference surface 1 c is to bemounted to another part while the sink surface 1 d is expected to sink.

[0122] Reference will be made to FIGS. 24A, 24B, 25A and 25B fordescribing a molding apparatus 2 for producing the above molding 1. Asshown, the molding apparatus 2 includes a stage 3 loaded with a lowermold 4. An upper mold 5 is positioned above the lower mold 4 and movableinto and out of contact with the lower mold 4 by being driven by aclamping device, not shown.

[0123] A plurality of (four in the embodiment) inserts are interposedbetween the lower mold 4 and the upper mold 5 and constitute cavityinserts. Specifically, mirror inserts 6 and 7 facing each other arerespectively formed with mirror surfaces 6 a and 7 a for forming themirror surfaces 1 a and 1 b of the molding 1. A reference insert 8 and asink insert 9 face each other at both sides of the mirror inserts 6 and7 and are respectively formed with non-transfer surfaces 8 a and 9 a inorder to form the reference surface 1 c and sink surface 1 d. Thesurfaces of the inserts 6-9 form a cavity 10. The non-transfer surfaces8 a and 9 a each is formed with fine irregularities or undulation.

[0124] It is to be noted that FIGS. 24A and 24B show only one half ofthe molding apparatus 2; the other half is also provided with cavityinserts identical with the cavity inserts 6-9. A sprue, not shown, isformed in the upper mold 5 while a sprue 6 b is formed in the mirrorinsert 6 and communicable to the above sprue. An injection moldingmachine, not shown, feeds molten resin to the cavity 10 via the sprue ofthe upper mold and sprue 6 b.

[0125] A vent hole 111 is formed in the sink insert 9. The vent hole 11is open to the cavity 10 at one end and connected to a feed tube 12 atthe other end. The feed tube 12 is interposed between the lower mold 4and the upper mold 5 and connected to a gas feed unit 14 via atemperature control unit 13. A gas, e.g., air compressed to apreselected pressure by the gas feed unit 14 and controlled to apreselected temperature by the temperature control unit 13 is fed viathe feed tube 12.

[0126] In the illustrative embodiment, molten resin heated above itssoftening point is injected into the cavity 10 of the mold assemblyheated to a temperature lower than the softening point of the resin.Therefore, the temperature control unit 13 controls the gas to atemperature about 3° C. lower than the temperature of the mirror inserts6 and 7 and reference insert 8. It follows that the temperature of thegas fed from the feed tube 11 to the sink surface 1 d is lower than thetemperature of the mirror surfaces 1 a and 1 b and reference surface 1c. The lower mold 4 and upper mold 5 surrounding the inserts 6-9 each isprovided with a temperature control mechanism including a heater and anoil cooler, not shown. The heater and oil cooler respectively heat andcool the associated molds 4 and 5 and therefore the inserts 6-9.

[0127] In the illustrative embodiment, the temperature control unit 13and gas feed unit 14 constitute a feeding device, and constitute gasfeeding means in combination with the vent hole 11.

[0128] The operation of the above arrangement will be describedhereinafter. When a lens or similar plastic optical element is producedby conventional injection molding, molding conditions allowing theentire area to be transferred (allowing the internal pressure of themolding to drop substantially to zero at the time of take-out) are setup. However, because molten resin is sharply cooled as soon as it isintroduced into a mold, the resulting temperature distribution, pressuredistribution, density distribution and so forth disturb the shape of amolding. This, coupled with the internal strain (deflection) of theresin, adversely influences the optical characteristic of the molding.Although the transfer of the mold configuration, internal strain anddeformation may be reduced if a molding is caused to partly sink, it isextremely difficult to specify the part of a molding to sink. Thisembodiment is significant in that it can specify the part of a moldingto sink, as follows.

[0129] While the mold assembly is held at a temperature lower than thesoftening point of resin, molten resin A heated above its softeningpoint is injected into the cavity 10. Then, a resin pressure is causedto act on the transfer surfaces 6 a and 7 a of the mirror inserts 6 and7, respectively. At the same time as the injection of the resin A, acool gas compressed to a preselected pressure by the air feed unit 14and controlled to a preselected temperature by the temperature controlunit 13 is fed to the sink surface 1 d via the vent hole 11. The feed ofthe gas is continued until the resin A has been fully injected into thecavity 10. At this instant, the sink surface 1 d lower in temperaturethan the mirror surfaces 1 a and 1 b solidifies first and increases itsviscosity. This makes it difficult for the sink surface 1 d to remain incontact with the non-transfer surface 9 a of the sink insert 9 beforethe end of the injection of the resin A. After the injection of theresin A and the following stop of feed of the cool gas, the cavity 10 iscaused to dwell at a preselected pressure and cooled. As soon as thepressure inside the cavity 10 drops substantially to zero, the uppermold 5 is opened away from the lower mold 4. Subsequently, the molding 1is taken out of the cavity 10.

[0130] The sink surface 1 d of the molding 1 obtains a parting abilityearlier than the other surfaces of the same. As a result, the sinksurface 1 d begins to sink earlier than the other surfaces contactingthe inserts 6-8. This successfully prevents the mirror surfaces 1 a and1 b from sinking and thereby allows the desired mirror surfaces 1 a and1 b to be faithfully transferred to the molding 1 in a short moldingcycle.

[0131] Moreover, the sink surface 1 d is held at a temperature lowerthan the temperature of the resin A from the end of the resin injectionto the beginning of cooling. Consequently, a temperature difference doesnot occur between the mirror surfaces 1 a and 1 b and the sink surface 1d during cooling, so that an internal strain is prevented from remainingin the molding after the opening of the mold assembly. This not onlyprevents the accuracy of the mirror surfaces 1 a and 1 b fromdecreasing, but also prevents the entire molding 1 from deforming.

[0132] In addition, the gas feeding means can be implemented only if thevent hole 11 is formed in the sink insert 9 and connected to thetemperature control unit 13 and gas feed unit 14. The mold assembly istherefore simple in construction.

16th Embodiment

[0133] Referring to FIGS. 26, 27A-27D and 28A-28D, a method and anapparatus for producing a plastic molding representative of a sixteenthembodiment of the present invention will be described. A molding to beproduced by this embodiment is identical in configuration with themolding of the fifteenth embodiment and will be described with referenceto FIGS. 23A-23C. Structural elements identical with the elements of thefifteenth embodiment are designated by identical reference numerals andwill not be described specifically in order to avoid redundancy.

[0134] As shown in FIGS. 26 and 27A-27D, a plurality of (four in theembodiment) inserts are interposed between the lower mold 4 and theupper mold 5 and constitute cavity inserts. Specifically, mirror inserts16 and 17 facing each other are respectively formed with mirror surfaces16 a and 17 a for forming the mirror surfaces 1 a and 1 b of a molding.A reference insert 18 and a sink insert 19 face each other at both sidesof the mirror inserts 16 and 17 and are respectively formed withnon-transfer surfaces 18 a and 19 a in order to form the referencesurface 1 c and sink surface 1 d of the molding. The surfaces of theinserts 16-19 form a cavity 20. The non-transfer surfaces 18 a and 19 aeach is formed with fine irregularities.

[0135] A sprue, not shown, is formed in the upper mold 5 while a sprue 6b is formed in the mirror insert 16 and communicable to the above sprue.An injection molding machine, not shown, injects molten resin into thecavity 20 via the sprue of the upper mold 5 and sprue 16 b. A vent hole21 is formed in the sink insert 19. The vent hole 21 is open to thecavity 20 at one end and connected to a feed tube 22 at the other end.The feed tube 22 is interposed between the lower mold 4 and the uppermold 5.

[0136] The feed tube 22 is connected to a gas feed unit 23. The gas feedunit 23 feeds a gas, e.g., air compressed to a preselected pressure tobetween the sink surface 1 d and the transfer surface 19 a via the feedtube 22 and vent hole 21. In this embodiment, the gas feed unit 23constitutes a feeding device and constitutes gas feeding means incombination with the vent hole 21 and feed tube 22.

[0137] The operation of the illustrative embodiment will be describedwith reference to FIGS. 27A-27D. As shown, while the mold assembly isheld at a temperature lower than the softening point of resin, moltenresin A heated above its softening point is injected into the cavity 20.Then, a resin pressure is caused to act on the transfer surfaces 16 aand 17 a of the mirror inserts 16 and 17, respectively. At the same timeas the injection of the resin A, a gas compressed to a preselectedpressure by the air feed unit 23 is fed to between the sink surface 1 dand the non-transfer transfer surface 19 a. The feed of the gas iscontinued until the resin A has been fully injected into the cavity 20(see FIGS. 27A and 27B). At this instant, a gas layer is formed betweenthe non-transfer surface 19 a and the sink surface 1 d, making itdifficult for the sink surface 1 d to remain in contact with thenon-transfer surface 19 a before the end of the injection of the resinA.

[0138] After the injection of the resin A and the following stop of feedof the gas, the cavity 20 is caused to dwell at a preselected pressureand cooled. As a result, the gas layer between the sink surface 1 d andthe non-transfer surface 19 a is compressed by the internal pressure ofthe resin A, but remains between them (see FIG. 27C). Such residual gasexpands as the internal pressure approaches zero, separating the sinksurface 1 d from the non-transfer surface 19 a. When the internalpressure reaches zero, the non-transfer surface 19 a obtains a partingability earlier than the other surfaces. When the pressure inside thecavity 20 drops substantially to zero, the upper mold 5 is released fromthe lower mold 4. Subsequently, the molding 1 is taken out of the cavity20.

[0139] In this manner, the sink surface 1 d begins to sink earlier thanthe other surfaces contacting the inserts 16-18. This successfullyprevents the mirror surfaces 1 a and 1 b from sinking and thereby allowsthe desired mirror surfaces 1 a and 1 b to be faithfully transferred tothe molding 1 in a short molding cycle. Moreover, the gas layer remainsbetween the sink surface 1 d and the non-transfer surface 19 a until thecooling step begins after the injection of the molten resin, preventingthe pressure difference between the mirror surfaces 1 a and 1 b and thesink surface 1 d from increasing during cooling. Consequently, theinternal strain of the molding 1 is prevented from remaining after theopening of the mold assembly. This not only prevents the accuracy of themirror surfaces 1 a and 1 b from decreasing, but also prevents theentire molding 1 from deforming.

[0140] This embodiment may be practiced with the same configuration asthe fifteenth embodiment, as follows. The gas fed from the gas feed unit23 is controlled to substantially the same temperature as the mold bythe temperature control unit 31 shown in FIG. 15. In this case, as shownin FIGS. 28A-28D, while the mold assembly is held at a temperature lowerthan the softening point of resin, molten resin heated above itssoftening point is injected into the cavity 20. Then, a resin pressureis caused to act on the transfer surfaces 6 a and 7 a of the mirrorinserts 6 and 7, respectively. At the same time as the injection of theresin, the gas compressed to a preselected pressure by the gas feed unit23 is fed to between the sink surface 1 d and the non-transfer surface 9a. The feed of the gas is continued until the resin A has been fullyinjected into the cavity 10 (see FIGS. 28A and 28B). At this instant, agas layer is formed between the non-transfer surface 9 a and the sinksurface 1 d, making it difficult for-the sink surface 1 d to remain incontact with the non-transfer surface 9 a before the end of theinjection of the resin.

[0141] After the injection of the resin and the following stop of feedof the gas, the cavity 10 is caused to dwell at a preselected pressureand cooled. As a result, the gas layer between the sink surface 1 d andthe non-transfer surface 9 a is compressed by the internal pressure ofthe resin, but remains between them (see FIG. 28C). The residual gasexpands as the internal pressure approaches zero, separating the sinksurface 1 d from the non-transfer surface 9 a. When the internalpressure reaches zero, the non-transfer surface 9 a obtains a partingability earlier than the other surfaces (see FIG. 28D). When thepressure inside the cavity 10 drops substantially to zero, the uppermold 5 is released from the lower mold 4. Subsequently, the molding 1 istaken out of the cavity 10.

17th Embodiment

[0142] A method and an apparatus for producing a plastic moldingrepresentative of a seventeenth embodiment of the present invention willbe described with reference to FIGS. 29, 30A and 30B. A molding to beproduced by this embodiment is identical in configuration with themolding of the fifteenth embodiment and will be described with referenceto FIGS. 23A-23C. Structural elements identical with the elements of thefifteenth embodiment will be designated by identical reference numeralsand will not be described specifically in order to avoid redundancy.

[0143] As shown, a plurality of (four in the embodiment) inserts areinterposed between the lower mold 4 and the upper mold 5 and constitutecavity inserts. Specifically, mirror inserts 26 and 27 facing each otherare respectively formed with mirror surfaces 26 a and 27 a for formingthe mirror surfaces 1 a and 1 b of a molding. A reference insert 28 anda sink insert 29 face each other at both sides of the mirror inserts 26and 27 and are respectively formed with non-transfer surfaces 28 a and29 a in order to form the reference surface 1 c and sink surface 1 d ofthe molding. The surfaces of the inserts 16-19 form a cavity 30. Thenon-transfer surfaces 28 a and 29 a each is formed with fineirregularities.

[0144] A sprue, not shown, is formed in the upper mold 5 while a sprue26 b is formed in the mirror insert 26 and communicable to the abovesprue. An injection molding machine, not shown, injects molten resininto the cavity 30 via the sprue of the upper mold 5 and sprue 26 b. Avent hole 31 is formed in the sink insert 29. One end of the vent hole31 is communicated to a gas feed unit 36 via a vent hole 32 formed inthe lower mold and a feed tube 34. The other end of the vent hole 31 iscommunicated to the outside of the mold assembly via a exhaust tube 35.

[0145] The gas feed unit 36 feeds gas, e.g., air controlled to apreselected pressure and a preselected temperature to the vent hole 31via the feed tube 34 and vent hole 32, and then discharges it via thevent hole 33 and exhaust tube 35. The gas therefore cools thenon-transfer surface 29 a of the sink insert 29.

[0146] In the illustrative embodiment, molten resin heated above itssoftening point is injected into the cavity 30 of the mold heated to atemperature lower than the softening point of the resin. Therefore, atemperature control unit 36 controls the temperature of the gas to atemperature about 3° C. lower than the temperature of the mirror inserts26 and 27 and reference insert 28. It follows that the temperature ofthe gas fed from the vent hole 31 to the sink surface 29 is lower thanthe temperature of the mirror surfaces 26 a and 27 a and non-transfersurface 28 a.

[0147] In this embodiment, the gas feed unit 36, feed tube 34, ventholes 31-33 and exhaust tube 35 constitute cooling means.

[0148] In operation, before the injection of molten resin, a coot gascontrolled to a preselected pressure and a preselected temperature isfed from the gas feed unit 36 to the non-transfer surface 29 a via thevent hole 31 so as to cool the non-transfer surface 29 a. Then, whilethe mold assembly is held at a temperature lower than the softeningpoint of resin, molten resin A heated above its softening point isinjected into the cavity 30. Subsequently, a resin pressure is caused toact on the transfer surfaces 26 a and 27 a of the mirror inserts 26 and27, respectively. The feed of the cool gas is continued until the resinA has been fully injected into the cavity 30. At this instant, the sinksurface 1 d lower in temperature than the mirror surfaces 1 a and 1 bsolidifies first and increases its viscosity, making it difficult forthe sink surface 1 d to remain in contact with the non-transfer surface29 a before the end of the injection of the resin A. After the injectionof the molten resin and the following stop of feed of the cool gas, thecavity is caused to dwell at a preselected pressure and cooled. When thepressure inside the cavity 30 drops substantially to zero, the uppermold 5 is released from the lower mold 4. Subsequently, the molding 1 istaken out of the cavity 20. This embodiment achieves the same advantagesas the fifteenth embodiment.

18th Embodiment

[0149] A method and an apparatus for producing a plastic moldingrepresentative of a seventeenth embodiment of the present invention willbe described with reference to FIGS. 31, 32A-32C, 33A, 33B, 34A-34C and35. A molding to be produced by this embodiment is identical inconfiguration with the molding of the fifteenth embodiment and will bedescribed with reference to FIGS. 23A-23C. Structural elements identicalwith the elements of the fifteenth embodiment will be designated byidentical reference numerals and will not be described specifically inorder to avoid redundancy.

[0150] As shown, a plurality of (four in the embodiment) inserts areinterposed between the lower mold 4 and the upper mold 5 and constitutecavity inserts. Specifically, mirror inserts 41 and 42 facing each otherare respectively formed with mirror surfaces 41 a and 42 a for formingthe mirror surfaces 1 a and 1 b of a molding. A reference insert 43 anda sink insert 44 face each other at both sides of the mirror inserts 41and 42 and are respectively formed with non-transfer surfaces 43 a and44 a in order to form the reference surface 1 c and sink surface 1 d.The surfaces of the inserts 4144 form a cavity 45. The non-transfersurfaces 44 a and 45 a each is formed with fine irregularities.

[0151] A sprue, not shown, is formed in the upper mold 5 while a sprue41 b is formed in the mirror insert 41 and communicable to the abovesprue. An injection molding machine, not shown, injects molten resininto the cavity 45 via the sprue of the upper mold 5 and sprue 41 b. Avent hole 46 is formed in the sink insert 44. The vent hole 46 is opento the cavity 45 at one end and connected to a bore 47 at the other end.The bore 47 is communicated to a flow rate control unit 50 via a venthole 48 formed in the lower mold 4 and a feed tube 49. The flow ratecontrol unit 50 is connected to a gas feed unit 53 via a pressurecontrol unit 51 and a temperature control unit 52.

[0152] The gas feed unit 53 constitutes a gas source. The temperaturecontrol unit 52 controls the temperature of a gas fed from the gas feedunit 53. The pressure control unit 51 controls the pressure of the gasfed from the gas feed unit 53. Further, the flow rate control unit 50controls the flow rate of the gas fed from the gas feed unit 53. Thevent hole 47 is communicated to an exhaust valve 56 via a vent hole 54formed in the lower mold 5 and an exhaust tube 55. The gas fed from thegas feed unit 53 to the vent hole 47 is discharged to the outside whenthe exhaust valve 56 is open, or introduced into the cavity 45 when thevalve 56 is closed.

[0153] In this embodiment, the flow rate control unit 50, pressurecontrol unit, temperature control unit 52 and gas feed unit 53constitute a feeding device. The feeding device constitutes gas feedingmeans in combination with the vent holes 46, 47, 48 and 54, feed tube49, and exhaust valve 56.

[0154] The operation of the illustrative embodiment will be describedwith reference to FIGS. 33A, 33B and 34A-34C. Briefly, this embodimentis characterized in that a step of pressing the sink surface 1 d ofmolten metal with the gas is combined with at least one of a step oflowering the temperature of the sink surface 1 d below the temperatureof the mirror surfaces 1 a and 1 b, a step of forming a gas layerbetween the sink surface 1 d and the sink insert 44, and a step oflowering the temperature of the sink insert 44 facing the sink surface 1d below the temperature of the mirror inserts 41 and 42. The followingdescription will concentrate on the combination of all of such steps.

[0155] First, the exhaust valve 56 is opened to feed a small amount ofgas to the vent hole 47 via the flow rate control unit 50, therebycooling the sink insert 44 (see FIG. 33A). Specifically, the flow rateof the gas is so selected as to prevent the gas from entering the cavity45; otherwise, the gas would enter the cavity 45 and cool even themirror surfaces 41 and 42. If desired, the temperature of the gas may becontrolled in order to promote the effective cooling of the sink insert44.

[0156] After the mold assembly has been heated to a temperature lowerthan the softening point of resin, but before molten resin heated to atemperature above its softening point is injected into the cavity 45,the flow rate and pressure of the gas are increased by the flow ratecontrol unit 50 and pressure control unit 51, respectively. As a result,the gas is admitted into the cavity 45. Subsequently, the molten resinbegins to be injected into the cavity 45 (see FIG. 33B). The increase inthe flow rate of the gas promotes the cooling of the resin while theincrease in the pressure of the gas allows the gas to press the sinksurface 1 d and allows a gas layer to be formed between the sink surface1 d and the sink insert 44.

[0157] After the injection of the resin (see FIG. 34A), the exhaustvalve 56 is closed while the pressure of the gas is adequatelycontrolled by the pressure control unit 51. As a result, the cavity 45is caused to dwell at a preselected pressure and cooled (see FIGS. 34Band 34C). When the pressure inside the cavity 45 drops substantially tozero, the upper mold 5 is released from the lower mold 4. Thereafter,the molding 1 is taken out of the cavity 45.

[0158] This embodiment achieves the same advantages as the fifteenthembodiment, and in addition achieves an advantages that the sink surface1 d is constantly pressed and therefore easily separates from the sinkinsert 44. This allows the sink surface 1 d to sink more positively.

[0159]FIG. 35 shows an alternative arrangement wherein the flow controlunit 50 is connected to a gas conduitwork 57 available in a factory.

[0160] Constantly pressing the sink surface 1 d, as shown and described,is not essential. Alternatively, there may be used at least one of threedifferent methods: lowering the temperature of at least one of thenon-transfer surfaces of the resin below the temperature of the transfersurfaces from the beginning to the end of the injection of the resin,forming a gas layer between at least one of the non-transfer surfaces ofthe resin and the mold assembly, and lowering the temperature of themold portion facing at least one of the non-transfer surfaces of theresin below the temperature of the mold portion facing the transfersurfaces.

[0161] The fifteenth to eighteenth embodiments shown and described havethe following unprecedented advantages.

[0162] (1) The non-transfer surface of a molding obtains a partingability earlier than the other surfaces of the same. This successfullyprevents the transfer surfaces of the molding from sinking and therebyallows desired mirror surfaces to be faithfully transferred to themolding in a short molding cycle.

[0163] (2) The non-transfer surface of molten resin is held at atemperature lower than the temperature of the transfer surfaces from theend of resin injection to the beginning of cooling. Consequently, atemperature difference does not occur between the transfer surfaces andthe non-transfer surfaces during cooling, so that an internal strain isprevented from remaining in the molding after the opening of a moldassembly. This not only prevents the accuracy of the transfer surfacesfrom decreasing, but also prevents the entire molding from deforming.

[0164] (3) A gas layer is formed between the non-transfer surface of theresin and the mold assembly until the cooling step begins after theinjection of the molten resin, preventing the pressure differencebetween the transfer surfaces and the non-transfer surface fromincreasing during cooling. Consequently, the internal strain of themolding is prevented from remaining after the opening of the moldassembly. This not only prevents the accuracy of the transfer surfacesfrom decreasing, but also prevents the entire molding from deforming.

[0165] (4) The gas layer is formed between the non-transfer surface ofthe resin and the mold and/or the temperature of the non-transfersurface is held lower than the temperature of the transfer surface untilthe cooling step begins after the injection of the molten resin, therebypreventing a difference in temperature or pressure between the transfersurface and the non-transfer surface from increasing during cooling.Consequently, the internal strain of the molding is prevented fromremaining after the opening of the mold assembly. This not only preventsthe accuracy of the transfer surfaces from decreasing, but also preventsthe entire molding from deforming.

[0166] (5) The temperature of the non-transfer surface of the resin islowered, the non-transfer surface is pressed, and/or the gas layer isformed between the non-transfer surface of the resin and the mold. Thisallows the non-transfer surface to sink with priority by use of a simpleconstruction.

[0167] (6) Gas feeding means can be implemented only if a vent hole isformed in the mold and communicated to a feeding device. This preventsthe mold configuration from being complicated.

[0168] (7) By cooling the non-transfer surface of the mold assembly withcooling means, it is possible to cool the non-transfer surface of theresin. The non-transfer surface can therefore be caused to sink by aninexpensive construction.

[0169] Various modifications will become possible for those skilled inthe art after receiving the teachings of the present disclosure withoutdeparting from the scope thereof. For example, while the embodimentshave concentrated on a molding in the form of a rectangular lens (havingtwo mirror surfaces or optical surfaces), the present invention issimilarly applicable to, e.g., a mirror having a single mirror surfaceor a prism having a plurality of mirror surfaces.

What is claimed is:
 1. In a molding produced by an injection moldassembly having a pair of molds including a mold surface forming acavity having a preselected volume, at least one transfer surface fortransferring a mirror surface formed on said mold surface to saidmolding, and a gate for filling said cavity with a molten material byinjection, and by injecting said molten material into said cavity viasaid gate and then cooling said molten material, said injection moldassembly includes at least one vent hole having a preselected openingarea, and at least one bore communicated to said vent hole for applyinga preselected air pressure to said molding, and a step portion formed onsaid mold surface between said vent hole and said transfer surface.
 2. Amolding as claimed in claim 1 , wherein either a pressure difference oran air pressure is generated between said transfer surface and a venthole portion of said molding facing said vent hole to thereby cause saidvent hole portion to sink.
 3. A molding as claimed in claim 2 , whereinsaid step portion is formed on said mold surface where said vent hole ispresent.
 4. A molding as claimed in claim 3 , wherein said step portionisolates said vent hole and said transfer surface.
 5. A molding asclaimed in claim 3 , wherein said step portion comprises a single stepsurrounding said vent hole.
 6. A molding as claimed in claim 3 , whereinsaid step portion comprise a single step similar in configuration to acontour of the mold surface where said vent hole is present.
 7. Amolding as claimed in claim 2 , wherein said step portion isolates saidvent hole and said transfer surface.
 8. A molding as claimed in claim 2, wherein said step portion comprises a single step surrounding saidvent hole.
 9. A molding as claimed in claim 2 , wherein said stepportion comprise a single step similar in configuration to a contour ofthe mold surface where said vent hole is present.
 10. A molding asclaimed in claim 2 , wherein said step portion comprises a projection.11. A molding as claimed in claim 2 , wherein said step portioncomprises a recess.
 12. A molding as claimed in claim 2 , wherein saidstep portion is provided on said transfer surface.
 13. A molding asclaimed in claim 12 , wherein said step portion comprises a pair ofsteps so configured as to sandwich a longitudinal surface of saidtransfer surface.
 14. A molding as claimed in claim 12 , wherein saidstep portion comprises a single step so configured as to surround saidtransfer surface.
 15. A molding as claimed in claim 2 , wherein saidstep portion comprises a pair of steps so configured as to sandwich alongitudinal surface of said transfer surface.
 16. A molding as claimedin claim. 2, wherein said step portion comprises a single step soconfigured as to surround said transfer surface.
 17. A molding asclaimed in claim 2 , wherein said step portion is tapered in a crosssection.
 18. A molding as claimed in claim 2 , wherein said step portionis triangular in a cross-section.
 19. A molding as claimed in claim 2 ,wherein said step portion is arcuate in cross section.
 20. A molding asclaimed in claim 2 , wherein said step portion has a height greater than0.1 mm inclusive.
 21. In an injection molding method for producing amolding by using a mold assembly having a pair of molds including a moldsurface forming a cavity having a preselected volume, at least onetransfer surface for transferring a mirror surface formed said moldsurfaces to said molding, and a gate for filling said cavity with amolten material by injection, and by injecting said molten material intosaid cavity via said gate and then cooling said molten material, saidmold surface is formed with, outside of said transfer surface, at leastone vent to hole having a preselected opening area and at least one borecommunicated to said vent hole for applying a preselected air pressureto said molding material, and the air pressure is continuously generatedvia said vent hole even after a pressure of said molding material insaid cavity has dropped to zero.
 22. A method as claimed in claim 21 ,wherein either a pressure difference or an air pressure is generatedbetween said transfer surface and a vent hole portion of said moldingfacing said vent hole.
 23. A method as claimed in claim 22 , wherein theair pressure is higher than an atmospheric pressure (about 0.1 MPa)inclusive, but lower than 2 MPa inclusive.
 24. A mold assembly having apair of molds including a mold surface forming a cavity having apreselected volume, at least one transfer surface for transferring amirror surface formed on said mold surface to said molding, and a gatefor filling said cavity with a molten material by injection, andinjecting said molten material into said cavity via said gate and thencooling said molten material, said mold surface is formed with, outsideof said transfer surface, at least one vent hole having a preselectedopening area and at least one bore communicated to said vent hole forapplying a preselected air pressure to said molding material, and atleast one exhaust hole located at a position adjoining said vent hole,but not facing said transfer surface.
 25. A mold assembly as claimed inclaim 24 , said exhaust hole surrounds said vent hole.
 26. A moldassembly as claimed in claim 24 , wherein said exhaust hole is similarin configuration to a contour of said mold surface where said draft holeis present.
 27. A mold assembly as claimed in claim 24 , wherein saidexhaust hole comprises a porous member.
 28. A mold assembly as claimedin claim 24 , wherein forced exhaustion means is communicated to saidexhaust hole.
 29. A mold assembly as claimed in claim 24 , wherein saidexhaust hole has an opening width of 0.001 mm to 0.5 mm.
 30. A method ofproducing a plastic molding, comprising the steps of: preparing a moldassembly including at least one transfer surface and at least onenone-transfer surface formed on a surface other than said transfersurface, said transfer surface and said non-transfer surface forming atleast one cavity; injecting molten resin heated to a temperature above asoftening point thereof into said cavity; causing a resin pressure toact on said transfer surface to thereby cause said resin to adhere tosaid transfer surface, and then cooling said resin to a temperaturebelow the softening point; opening said mold assembly in order to allowa resulting molding to be taken out; and lowering a temperature of atleast one non-transfer surface of said resin below a temperature of saidresin on said transfer surface during an interval between a beginningand an end of injection of said resin into said cavity.
 31. A method ofproducing a plastic molding, comprising the steps of: preparing a moldassembly including at least one transfer surface and at least onenone-transfer surface formed on a surface other than said transfersurface, said transfer surface and said non-transfer surface forming atleast one cavity; injecting molten resin heated to a temperature above asoftening point thereof into said cavity; causing a resin pressure toact on said transfer surface to thereby cause said resin to adhere tosaid transfer surface, and then cooling said resin to a temperaturebelow the softening point; opening said mold assembly in order to allowa resulting molding to be taken out; and forming a gas layer between atleast one non-transfer surface of said resin and said mold assemblyduring an interval between a beginning and an end of injection of saidresin into said cavity.
 32. A method of producing a plastic molding,comprising the steps of: preparing a mold assembly including at leastone transfer surface and at least one none-transfer surface formed on asurface other than said transfer surface, said transfer surface and saidnon-transfer surface forming at least one cavity; injecting molten resinheated to a temperature above a softening point thereof into saidcavity; causing a resin pressure to act on said transfer surface tothereby cause said resin to adhere to said transfer surface, and thencooling said resin to a temperature below the softening point; openingsaid mold assembly in order to allow a resulting molding to be takenout; and maintaining a portion of said mold assembly facing at least onenon-transfer surface of said resin lower in temperature than a portionof said mold assembly facing said transfer surface until injection ofsaid resin into said cavity ends.
 33. A method of producing a plasticmolding, comprising the steps of: preparing a mold assembly including atleast one transfer surface and at least one none-transfer surface formedon a surface other than said transfer surface, said transfer surface andsaid non-transfer surface forming at least one cavity; injecting moltenresin heated to a temperature above a softening point thereof into saidcavity; causing a resin pressure to act on said transfer surface tothereby cause said resin to adhere to said transfer surface, and thencooling said resin to a temperature below the softening point; openingsaid mold assembly in order to allow a resulting molding to be takenout; and effecting, during an interval between a beginning and an end ofinjection of said resin into said cavity, at least one of lowering atemperature of at least one non-transfer surface of said resin below atemperature of said resin on said transfer surface, forming a gas layerbetween at least one non-transfer surface of said resin and said moldassembly, and lowering a temperature of a portion of said mold assemblyfacing at least one non-transfer surface of said resin below atemperature of a portion of said mold assembly facing said transfersurface.
 34. A method of producing a plastic molding, comprising thesteps of: preparing a mold assembly including at least one transfersurface and at least one none-transfer surface formed on a surface otherthan said transfer surface, said transfer surface and said non-transfersurface forming at least one cavity; injecting molten resin heated to atemperature above a softening point thereof into said cavity; causing aresin pressure to act on said transfer surface to thereby cause saidresin to adhere to said transfer surface, and then cooling said resin toa temperature below the softening point; opening said mold assembly inorder to allow a resulting molding to be taken out; effecting, during aninterval between a beginning and and end of injection of said resin intosaid cavity, at least one of lowering a temperature of at least onenon-transfer surface of said resin below a temperature of said resin onsaid transfer surface, forming a gas layer between at least onenon-transfer surface of said resin and said mold, and lowering atemperature of a mold portion facing at least one non-transfer surfaceof said resin below a temperature of a mold portion facing said transfersurface; and pressing at least one non-transfer surface of said resin bya gas.